Mobile telephone use has evolved into a critical communication link for information flow. Originally, mobile telephones were capable of transmitting and receiving only speech signals. The proliferation of data transmission, such as facsimile and computer information, made it necessary for mobile telephones to be capable of both speech and data communication. This additional capability has been achieved primarily through the use of digital telephone network that are capable of converting analog speech signals to digital signals and combining them with the digital data signals. Examples of such digital telephone networks are the Pan-European digital mobile system Groupe Special Mobile (GSM) and a proposed U.S. system. The mobile digital telephone systems typically employ a type of time division multiplexing or "burst" communication where each burst contains several bits of digital information arranged according to a frame format defined in the standard for the system. Frames of information are divided into time slots, with each time slot being one or to more bits. The information conveyed in the time slots can be speech, data, or control signals.
The control signals determine whether a speech-only call or a speech and data call will be established between the mobile telephone and the mobile telephone system. In a speech-only call, the mobile telephone need only extract the appropriate time slot information designated as speech information according to the frame format. In a speech and data call, the mobile telephone must extract the appropriate signals from the speech time slots as well as from the data time slots. The received speech or speech and data information must then be stored in memory for processing by the mobile telephone. It is thus seen that a speech-only mobile telephone requires less memory than a speech and data telephone. A mobile telephone's memory is costly and consumes considerable power, thus, there is a need for mobile telephone circuit arrangements that provide speech or speech and data capabilities while at the same time reducing cost, power consumption, and complexity.
Because not all mobile telephone users require data capabilities, one possible solution to the above mentioned disadvantages is to offer two types of mobile telephones, a speech-only version and a speech and data version. In so doing, the speech-only version is not burdened with the added complexity, cost, and power consumption needed to provide data capabilities. The purchaser of a voice only mobile telephone who later desires to add data capabilities must discard his speech-only telephone and purchase a speech and data version, thereby wasting his initial capital investment.